The present invention relates generally to radio or wireless communications and, more particularly, relates to a multi-band receiver having multi-slot capability.
Wireless or radio frequency (RF) communication systems are an integral component of the ongoing technology revolution and are evolving at an exponential rate. Many wireless communication systems are configured as xe2x80x9ccellularxe2x80x9d systems, in which the geographic area to be covered by the cellular system is divided into a plurality of xe2x80x9ccellsxe2x80x9d. Mobile communication devices or stations (e.g., wireless telephones, pagers, personal communications devices and the like) in the coverage area of a cell communicate with a fixed base station or transmitter within the cell. Low power base stations are utilized, so that frequencies used in one cell can be re-used in cells that are sufficiently distant to avoid interference. Hence, a cellular telephone user, whether mired in traffic gridlock or attending a meeting, can transmit and receive phone calls so long as the user is within a cell served by a base station.
The communication format used in most wireless communications systems is a high-frequency carrier waveform modulated by low frequency or xe2x80x9cbasebandxe2x80x9d audio or data signals. Mobile stations (wireless handsets, for example) within a wireless communication system typically have a transmitter that xe2x80x9cmodulatesxe2x80x9d baseband signals (e.g., speech detected by the handset microphone) onto the carrier waveform. Amplitude modulation (AM) and frequency modulation (FM) techniques, for example, are well known to those of ordinary skill in the art. Mobile stations also typically have a receiver that xe2x80x9cdemodulatesxe2x80x9d the carrier waveform to extract the baseband signal. The carrier waveforms required for modulation and demodulation are high frequency, periodic waveforms and are typically generated by oscillators within the transmitter and receiver.
The available frequency spectrum is distributed among the cellular base stations according to a frequency plan. In a GSM network, for example, the transmission band covers 880-915 MHz and the receiving band covers 925-960 MHz (see detailed description herein and FIGS. 2-3). The transmit and receive bands, in turn, are partitioned into 200 kHz frequency channels. Many of these channels, obviously, are reserved for the actual transmission and reception of speech or data. These channels are known as xe2x80x9ctrafficxe2x80x9d channels. Other channels are reserved for control and monitoring operations. These channels are known as xe2x80x9cbroadcastxe2x80x9d channels. Information may be exchanged in slots tuned to broadcast channels about, for example, whether a hand over to a neighboring cell should be performed, or which traffic channel the mobile station should tune to next.
GSM networks use a time-division multiple access (TDMA) architecture. Each channel (within the transmit and receive bands) is available to multiple users, but at different times. In one TDMA implementation, each channel is subdivided in time into eight time slots. Hence, each frequency channel will be available to eight different transceivers or users at different times. Each time slot has a duration of approximately 0.577 ms (577 xcexcs), and eight time slots form a TDMA xe2x80x9cframexe2x80x9d, having a duration of 4.615 ms.
An example TDMA frame 80 having eight time slots 0-7 is illustrated in FIG. 1a. Frame 80 represents the channel/time assignments for one wireless device or mobile station. Slot zero is a traffic slot in which data is transmitted over a traffic channel, and slot three is also a traffic slot in which data is received over a traffic channel. Time slot six has been reserved as a monitor slot. In monitor slot six, the wireless device may monitor a broadcast channel for operational/control data, an adjacent cell for power information, or any other frequency on which control or operational data is exchanged.
Since the device must be tuned to different frequency channels in the traffic and monitor slots, a certain amount of resting time is required to permit retuning and settling of the oscillator. The unused or xe2x80x9crestxe2x80x9d time slots (slots 1, 2, 4 and 5 in frame 80, for example) between traffic and monitor slots are used to retune and settle the local oscillator to the next frequency channel. The tuning and settling time required in a conventional GSM receiver, in order to guarantee reprogramming of the oscillator over the frequency range extremes and locking within 100 Hz, is in the range of 840 ms (about 1.5 slots). In a frame structure such as frame 80, where the traffic and monitor slots are single and spaced apart, ample rest slots are available to accommodate this requirement.
One of the current development trends in GSM systems is xe2x80x9cmulti-slotxe2x80x9d reception and transmission. In multi-slot operation, a mobile station transmits and/or receives in multiple time slots within each TDMA frame. This is in contrast to the configuration of frame 80, in which there is only one receive and one transmit slot per frame. A multi-slot framework, by providing more receive and/or transmit slots per frame, drastically increases data transmission rates. Increased data transmission rates are particularly important for data-intensive applications such as wireless Internet access.
The present invention is directed to a mobile station having multi-slot reception capability. A TDMA frame 85 having multiple traffic and monitor slots within the receive band is shown in FIG. 1b. TDMA frame 85 has one transmit traffic slot zero, two receive traffic slots two and three, and three monitor slots four through six. Many other multiple slot assignments are of course possible. Frame 85 is just one possibility that is presented for exemplary purposes only.
Use of multiple slots is problematic in conventional receivers as there is often insufficient time to retune and settle the local oscillator when switching frequencies between traffic and monitor slots. In FIG. 1b, for example, there is only one slot available to switch from the transmit traffic channel to the receive traffic channel, and no slots are available to make the switch from the receive traffic channel to the receive monitor channel. For the latter frequency change, where adjacent slots are assigned to different frequency channels, the required oscillation frequency change must be effected virtually simultaneously.
Various attempts have been made to decrease the frequency channel switching time in order to accommodate multi-slot reception. One approach has been to increase the corner frequency of the receiver""s PLL loop filter in order to decrease the lock time of the PLL. This approach, however, decreases the utility of the filter and makes it difficult to meet spurious performance requirements. Another approach is to use fractional-N type architectures, which permit use of higher PLL reference and corner frequencies. Fractional-N architectures, however, still pass high levels of phase noise and, though they provide quicker lock times, they do not provide the speed that is necessary for instantaneous switching. Another approach is to simply use separate high frequency oscillators in the receiver for the traffic and monitor channels. This approach, though providing the necessary speed, is too costly to make its implementation feasible.
The present invention provides a multi-band receiver for a wireless communication device that has enhanced multi-slot reception capability. The inventive receiver provides virtually instantaneous frequency channel switching by mixing a pre-existing oscillation resource with an added low frequency oscillator. The added oscillator and mixer elements are inexpensive, can be integrated with the pre-existing components on one IC, and contribute a minimal amount of current drain to the receiver.
In one embodiment of the present invention, a receiver is provided. An antenna receives an Rx signal, and a first oscillator generates a relatively high frequency traffic LO signal. Second and third oscillators generate first and second relatively low frequency signals. A first mixer mixes the first and second relatively low frequency signals to produce a relatively high frequency monitor LO signal. A second mixer mixes either the traffic or monitor LO signal with the Rx signal to produce a first IF signal. A third mixer mixes the first IF signal and the second relatively low frequency signal to produce an output signal.
In one implementation, the receiver is a dual band receiver and the Rx signal is within a 925-960 MHz bandwidth for GSM operation or an 1805-1880 MHz for DCS operation. The traffic LO signal is within a 1325-1380 MHz (GSM) or 1405-1480 MHz (DCS) bandwidth. The first relatively low frequency signal is within a 554.2-589.2 MHz (GSM) or a 634.2-709.2 MHz (DCS) bandwidth, and the second relatively low frequency signal has a fixed frequency of 770.8 MHz. In a further implementation, a switch is provided for switching between the traffic and monitor LO signals.
Another embodiment of the present invention provides a wireless communication device. The device includes a microphone for capturing audio acoustic signals and converting the acoustic signals into electric signals, and a speaker for converting electric signals into audio acoustic signals. An antenna is provided for wireless transmission and reception of acoustic or data signals, and a transmitter is provided for transmitting acoustic or data signals over the antenna. A processor directs the overall operation of the device.
The communication device also includes a receiver. The receiver has a high frequency oscillator that generates a traffic signal and two low frequency oscillators. A first mixer mixes the signals from the two low frequency oscillators to generate a monitor signal. A second mixer mixes a received signal from the antenna with either the traffic or monitor signal from a switch to generate an IF signal. A third mixer mixes the IF signal with a signal from one of the low frequency oscillators to generate an output signal. In one implementation, the IF signal is mixed with a fixed low frequency signal.
The present invention also provides a method for wireless, multi-slot signal reception. The method includes the steps of:
generating a relatively high frequency traffic LO signal;
generating a first relatively low frequency signal;
generating a second relatively low frequency signal;
mixing the first and second relatively low frequency signals to generate a relatively high frequency monitor LO signal;
receiving an Rx signal in a traffic slot or a monitor slot of a TDMA frame;
mixing the Rx signal with the traffic LO signal if the Rx signal is received in the traffic slot to generate a first IF signal;
mixing the Rx signal with the monitor LO signal if the Rx signal is received in the monitor slot to generate the first IF signal; and
mixing the first IF signal with the one of the low frequency signals to generate an output signal.
Objects and advantages of the present invention include any of the foregoing, singly or in combination. Further objects and advantages will be apparent to those of ordinary skill in the art, or will be set forth in the following disclosure.